Synthesis, Characterization, and Variable-Temperature NMR Studies of Silver(I) Complexes for Selective Nitrene Transfer

Abstract

An array of silver complexes supported by nitrogen-donor ligands catalyze the transformation of C═C and C-H bonds to valuable C-N bonds via nitrene transfer. The ability to achieve high chemoselectivity and site selectivity in an amination event requires an understanding of both the solid- and solution-state behavior of these catalysts. X-ray structural characterizations were helpful in determining ligand features that promote the formation of monomeric versus dimeric complexes. Variable-temperature H and DOSY NMR experiments were especially useful for understanding how the ligand identity influences the nuclearity, coordination number, and fluxional behavior of silver(I) complexes in solution. These insights are valuable for developing improved ligand designs.

Rigidifying Ag(I) Complexes for Selective Nitrene Transfer

“…A ( S,S,R )‐( α ‐Me)‐ anti ‐Py 3 Pip 3 ligand was ideally suited to meet these conditions (Figure ); the relative ( S,S,R ) stereochemistry between the two chiral carbons of the piperidine and the α‐Me carbon of a single enantiomer of 2 were determined by single X‐ray crystallography . Formation of (α‐Me)‐ anti ‐Py 3 PipAgOTf 2 from 3 and an equimolar amount of AgOTf gave a one‐dimensional polymer in the solid state . However, VT‐NMR studies show that the resting state of 2 in solution is monomeric in the temperature ranges where NT is typically carried out (Figure ) .…”

“…A second feature that is both helpful and vexing is the tendency of Ag(I) complexes to exhibit dynamic behaviors in solution . While this enables significant latitude in the coordination environment around the putative nitrene intermediate, an in‐depth understanding of the solution‐state behavior of these complexes is key to the rational design of new ligands for site‐selective C−H amination . In particular, the ability to influence the chemo‐ and site‐selectivity of NT without changing the mechanism of the reaction was intriguing to us.…”

“…Improved site‐selective C−H bond amidation should be aided by minimizing the fluxional behavior of the silver complex, as well as enforcing a tridentate N‐coordination of the ligand to the metal. A ( S,S,R )‐( α ‐Me)‐ anti ‐Py 3 Pip 3 ligand was ideally suited to meet these conditions (Figure ); the relative ( S,S,R ) stereochemistry between the two chiral carbons of the piperidine and the α‐Me carbon of a single enantiomer of 2 were determined by single X‐ray crystallography . Formation of (α‐Me)‐ anti ‐Py 3 PipAgOTf 2 from 3 and an equimolar amount of AgOTf gave a one‐dimensional polymer in the solid state .…”

Synthesis, structures, photophysical properties, and catalytic characteristics of 2,9‐dimesityl‐1,10‐phenanthroline (dmesp) transition metal complexes

“…It is important to note that the ability to chelate metal ions also enables the utilization of phenanthroline complexes in analytical applications such as electrochemical, fluorometric, and bioorganic sensors . Many metal complexes of phenanthroline and closely related ligands have also found applications in catalytic processes such as intramolecular C–C coupling, transfer hydrogenation of ketones and nitriles, acyloxyalkylation of styrene derivatives, selective nitrene transfer, asymmetric aldol reactions, and in the synthesis of metal–organic frameworks for tandem catalytic C–H activation reactions . Phenanthroline‐based metal ion catalysts are also used to produce linear α‐olefins in the range of C 4 –C 20 , which are important industrial starting materials for plasticizers, detergents, and as co‐monomers for linear low‐density polyethylene (LLDPE) .…”

Silver(I) and Copper(I) Complexes of Semi‐Bulky Nitrogen‐Confused C‐Scorpionates

“…Aziridines are important synthetic intermediates in organic synthesis, medicinal chemistry, and polymer chemistry, Catalytic approaches to their syntheses have rapidly expanded in recent years , . Transition metal‐catalyzed reactions involving nitrene transfer to alkenes have gained prominence in this regard , . Of these latter catalysts, copper(I) and silver(I) scorpionate complexes [tris(pyrazolyl)borates, (normal scorpionates, or Tp R , Scheme , top left), tris(pyrazolyl)methanes (Tpm R or C ‐scorpionates, Scheme , top right), certain C‐heteroscorpionates (Scheme , bottom left), and certain nitrogen‐confused C‐scorpionates (Scheme , bottom right)] have proven adept at catalytically affording aziridines from alkenes and various nitrene sources under mild conditions.…”